Getting Started
Open the terminal and run this sequence of commands:
sudo -s
gksu gedit /etc/modprobe.d/ath9k.conf
And at the end of the file, insert this line:
options ath9k nohwcrypt=1
Save your file and exit. Reboot now your system, then test again your WiFi connection to see if its speed is resolved or not.
Source: http://www.upubuntu.com/2011/11/how-to-fix-slow-wifi-connection-on.html